Wednesday, November 6, 2013

Warning Shot: The Cheliabinsk Impactor

By Milton Garces, 6 November 2013

The Cheliabinsk (Russian) Meteor startled the world on 15 February 2013 and reminded us how quickly the assumption of continuity can be disrupted. The United Nations promptly formed an Action Team on Near-Earth Objects (1) and constructed a plan for the formation of an International Asteroid Warning Network, which was approved in November 2013 by the UN General Assembly.

That was quick. I suppose a ½ Megaton (Mt) fireball (1 Mt nuclear equivalent) blasting above a (once secret) Soviet nuclear weapons complex in the Ural Mountains could be misinterpreted (Figure 1).

Figure 1. Mayak nuclear complex in Northern Chelyabinsk (

Yet it was a rather fortuitous event in many ways. First, although many were unfortunately hurt, nobody was killed. As the UN wisely surmised, this was a warning shot and we should be better prepared next time. Second, it happened over a well-populated area where car-mounted cameras are ubiquitous, yielding an abundance of time-stamped video data. Third, we were all looking to the skies in anticipation of a completely unrelated Near-Earth Object (NEO), which most of us have already forgotten about. And fourth, this is an era of real-time geophysical monitoring, with global network of infrasound sensors ideally designed for the capture of massive airbursts.

Astronomers and geoscientists were quick to respond. Dash-cam videos, preliminary observations, results and analyses were distributed within hours of the event by citizens, individual researchers, NASA, IRIS, and the CTBTO (amongst others). The first analyses were out before the infrasound reached Antarctica (2), and numerous papers have already been published on this event.

Two letters in 06 November 2013 electronic edition of Nature revisit in detail the trajectory and explosive yield estimates for the Cheliabinsk Meteor. And this is where I do a full disclosure: I am a co-author on one of these Nature letters (3), but don’t let that lead you into thinking I know all about it. I’m number 16 in the 33-author list, and my minor contribution to this paper was the estimation of the infrasonic energy recorded at the nearest station in Kazakhstan, which at best confirmed other estimates. And I did not contribute at all to the Letter on the trajectory (4). But I have had the pleasure of learning from my colleagues and to revisit my old Space Science roots. And I also have an unfair advantage, as I have a bit of a head start.

But this is a fleeting advantage. The Cheliabinsk event is so important, it will take years for some of us to complete our studies. It is so big, and rocked Earth’s atmosphere so hard, that there is a special Natural Hazards session (5) on the Chelyabinsk Meteor at the Fall 2013 American Geophysical Union meeting in San Francisco (which I am co-chairing, since I’m in disclosure mode). I’m obviously invested in this event, and I anticipate it will influence my R&D agenda for the next 5-10 years. Here I concentrate on the Nature Letters because I know how much work has gone into them and that many of us are keen to use the chronology and trajectory results in Borovicka et al. (2013).

Let’s do a quick review the physics of an asteroid airburst. Let’s consider largish solid space rocks with substantial penetration depth, as the smaller and softer rocks usually burn up high. Since Earth is essentially plowing into most of these asteroids, they are coming into the atmosphere hot and fast, usually at hypersonic speeds. Thus their Mach cones are more cylindrical than conical, and as they rip into the atmosphere they generate intense low-frequency sound (infrasound). Their entry trajectories are often steep relative to the ground, but not always, as in the case of Chelyabinsk. The acoustic shock wave (airbust) impact on the ground will depend on the asteroid kinetic energy and the height of energy release. Their explosive airbust magnitude is often estimated by assuming all the kinetic energy is converted to explosive energy in Joules, which can be expressed in Megatons (Mt) of TNT equivalent by using 1 Mt ~ 4.2 x 10^15 J.

Figure 2. Left: Type 1 airburst (Tunguska type). Right: Type 2 airburst (Libyan Desert Type, Bucharest 5). From Boslough (2013a).

Boslough (2013a) proposed an airburst scale for hazard assessment and early warning of asteroid impactors based on a 1-5 rating, as in hurricane scales. Since this scale was first proposed at the 2011 Planetary Defense Conference (7) in Romania, it is referred to as the Bucharest scale.

Proposed Bucharest Airburst Warning Scale (Boslough, 2013a)

“1. High-altitude airburst with no possible damage. Bright light in sky followed by sonic boom. No recommended action.
2. High-altitude airburst with minor damage. Possible hazard from broken windows and dust from sonic boom shaking of structures. Recommended action: avoid standing near windows and anticipate respiratory hazard from dust in buildings. 2008 TC3 would have probably been this class.
3. High-altitude airburst with major damage. Possible hazard from many broken windows and unsecured structures like trailers blowing down due to blast wave. Recommended action: take cover in basements or strong structures. Consider leaving area.
4. Low-altitude airburst with heavy blast damage: Tunguska- class event. Structures within blast zone destroyed. Recommended action: evacuate blast zone and take cover outside that zone.
5. Low-altitude airburst with heavy thermal damage: Libyan Desert Glass class event. Fireball zone surrounded by blast zone. Everything within fireball zone incinerated, everything within blast zone blown down. Recommended action: evacuate fireball and blast zones, and take cover outside those zones.”

I apply this scale to a couple of meteors and a couple of atmospheric nuclear tests. The Bucharest scale does not apply to nukes, but it is useful to have a reference in Mt from better-calibrated point-source nuclear detonations. But even for man-made explosives, yield estimates can be easily off by a factor of two due to incomplete detonation or focusing, and in this narrative I will not belabor the difference between tonnes and tons or high vs nuclear explosive yields. Brown et al. (2013) provide the estimated yield of 0.5 Mt for the Chelyabinsk meteor using various methods, and below is a list of historical airburst events with increasing order of magnitude yields.

Nuke: Hiroshima  (1945), 0.6 km detonation height, Bucharest 5, Yield of 0.01 Mt

Meteor: Chelyabinsk Meteor (2013), 30 km detonation height, Bucharest 3, Yield of 0.5 Mt
Meteor: Tunguska Meteor (1908), 10 km detonation height, Bucharest 4, Yield of 5 Mt.

                                                      (Boslough, personal communication)
Nuke: Tsar Bomba (1961), 4 km detonation height, Bucharest 5, Yield of 50 Mt.

The lower detonation height of the Hiroshima explosion may be the cause of a higher Bucharest scale, and the sheer brutal massiveness of the Tsar Bomba guarantees a high Bucharest scale rating. The impactor of the postulated Chicxulub (Yucatan) Mass Extinction Event (66 mya) had an estimated diameter of 10 km and a yield of ~10^8 Mt (8). The Bucharest scale is not applicable, and neither should the TNT yield equivalence for an event of this scale. The ~1023 Joules of destructive energy released during this ground impact event has not been witnessed by humanity, and it is statistically unlikely that we will for a long while. The odds appear to be in our favor: the probability of Earth getting hit by an impactor with a diameter greater than ½ km is much less than one-in-a-million per decade (9). However, it is difficult to build reliable statistical models with small sample populations, and sometimes we just need to collect more and better data before we can make accurate statistical predictions.

This is one of the points of the Nature Letter by P. Brown et al. (3), who inferred a Chelyabinsk asteroid diameter of ~20m with (4) and used a combination of seismic, infrasound, satellite, and video observations to derive the 0.5 +/- 0.1 Mt yield estimate. He also inferred an absolute astronomical magnitude of -28 for its brightest stage, which may be dim 10 parsecs away but was 30 times brighter than the Sun to those poor souls directly below it. The peak energy deposition was at a height of ~30 km, where it radiated at a rate of ~80 kt/km (~3.4 x 10^14 J/km) of altitude. A better experimental scenario could not have been designed for an infrasound calibration explosion, as the peak airburst energy was deposited in the midst of the stratospheric waveguide, ensuring circumglobal propagation of infrasonic signals.

Based on historical studies, the chances of getting another Chely-sized or larger event in the next 20 years is ~13%. Which is fairly high, essentially an 8-bullet Russian roulette over the next couple of decades. More interesting is that asteroid impactors in the 10-50 m diameter range, as inferred from infrasound and satellite observations, may be more abundant than expected from the statistical models derived from telescopic and lunar cratering surveys. More impact data from such big meteors would be needed to demonstrate that this trend is real. Then again, if it is, we should be seeing and hearing more of these Megaton-yield asteroids over the next decades.

The Letter by Borovicka et al. (4) is delightful in its use of YouTube videos to carefully constrain the entry, fragmentation, and afterglow of the Chelyabinsk asteroid. Their forensic analyses provide a careful chronology of when and how the rocks fragmented, and where they landed. It is and excellent example of the effective use of existing ubiquitous sensing technology, in this case from the dash-cams and other video recording equipment popular in that region.

Figure 3. “Extended Data Figure 3: Deviation of fragment F1 from the main trajectory. Frame from video15. The time is counted from 3:20:20UT. The labelled marks identify points on the main trajectory at the given altitude (in kilometres). E represents the endpoint of the main trajectory.” From (4).

Figure 4. Ground projection of the terminal part of the bolide trajectory and meteorite-strewn field. Main trajectory (thick red line) and trajectory of fragment F1 (thin orange line) as plotted on Google Earth. The marks denote altitudes in kilometres. The predicted impact positions of 11 observed fragments (F1–F4, F6, F7 and F11–F15).” From (4).

This work provides a valuable chronology and includes the “dark flight” transect, the section of the trajectory after luminous flight ceases. The remaining luminous dust trail provides a Mach cylinder radius estimate of ~ 1km above 40 km, when a dense rock of ~20 m diameter and a mass of at least 10^6 kg slammed into the atmosphere at a speed of ~19 km/s. By the time it reached the height of 22 km, the main body had been reduced to 10^4 kg, and below 17 km it had broken up into numerous pieces (Fn), the largest fragment (F1) was estimated at 4.5 x 10^2 kg, which was fairly close to the actual 6 x 10^2 kg rock fished out of the lake. The Borovicka et al. (4) reference will be of value to anybody interested in reconstructing or refining the Chely entry trajectory at high spatial and temporal resolutions.

As previously noted, we have a ~13% chances of hearing about another Chely-type event in the next two decades (3), but the odds could be higher. Surprising high-yield asteroid airbursts and associated geopolitical misunderstandings could be avoided with increased vigilance. The recent UN Asteroid Warning initiative (1) builds on a Planetary Defense community that is ready to head off to space to nudge or nuke incoming asteroids (7). Boslough (9) recently published a decision-making system to assess what action to take in case of an incoming asteroid. For small objects, there is not much risk and we all can head out to do some fun field work with our favorite instruments (green zone, Figure 5). Once incoming space rocks get sizeable, our decisions are determined by how much lead time we have. With enough time we can nudge asteroids to a new orbit. If the object is large and we have over a year to prepare, we could exercise the nuclear option and blast it. But if the object is large and we don’t have enough time, our remaining choice is to brace for impact, shown in red as the “pray for a miracle” area (Figure 5).

Figure 5. Decisions support volume with 200-m asteroid size plane, and 10-year time plane. From Boslough, 2013b.

I’m glad somebody has thought this through, and that there is an international response plan involving rockets, space travel, and explosives. I humbly recommend the addition of high-power lasers, and assume robots are already included. Since I am personally averse to the red “zone of despair” in Figure 5, I fully support the UN Asteroid Warning initiative and the Planetary Defense community’s aims of figuring out how to detect and deflect large asteroids from hitting Earth. In the meantime, us ground-pounding geoscientists will keep collecting asteroid impact data with increasing temporal and spatial resolution to help us better prepare for (and respond to) unwelcome aggressive visitors from space. 

1. Threat of space objects demands international coordination, UN team says (20 February 2013).
2. Antarctic Sound Check (25 February 2013).
3. Brown P. et al. (2013). A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors, Nature, doi:10.1038/nature12741.
4. Borovicka J. et al. (2013). The trajectory, structure, and origin of the Chelyabinsk asteroidal impactor, Nature, doi:10.1038/nature12671.
5. Fall AGU Meeting, Tuesday 10 December 2013. Session NH23. The Chelyabinsk Meteor Event.
6. Boslough, M. (2013a) Airburst warning and response, Acta Astronautica, 10.1016/j.actaastro.2013.09.007.
7. 2011 Planetary Defense Conference, Bucharest, Romania,
8. Covey, C., S. L. Thompson, P. R. Weissman, M. C. MacCracken (1994).
Global climatic effects of atmospheric dust from an asteroid or comet impact on Earth
Global and Planetary Change, Volume 9, Issues 3–4, December 1994, Pages 263–273
9. Boslough, M. (2013b), Impact decision support diagrams, Acta Astronautica, 10.1016/j.actaastro.2013.08.013.

Tuesday, May 21, 2013

May Musings, 2 AM

It has been work mayhem over the last five months, but there is a sliver of light in the distance. Submit new papers, close lingering contracts, pursue new work, and charge ahead towards the new world order. It's been sad to lose friends and colleagues to greed during these uneasy times of sequestration, but there is constructive disillusion in the revelation of their character under perceived duress.

I find it a bit comical for scientists to justify dishonorable and unethical actions under the guise of "survival": what we do as professionals is far removed from the high risk and bloody imminent demise of survival situations. Avarice, laziness, even envy I can understand, but "survival" is either subterfuge or delusion. Either of those two yields a loss of credibility, which is the main, if not the only, asset we sell as scientists.

But beyond comical is to explain away objectionable actions with the cliche "it's not personal, it's just business", a line from The Godfather associated with irrefutable offerings, concrete shoes, and general thuggery. Scientists, pay heed: this jaded line in no way makes you more professional or businesslike, and should be avoided unless delivered in irony or jest.

I have observed through my travels that Art and Science are the fruits of stable, evolved societies, and best flourish once the foundations of law, economy, medicine, and engineering are firmly established. In other words, the Arts and Sciences are the fruits of abundance, of surplus. To be a research scientist is a privilege and a call to act according to the high principles of ethics and reason inherent to the scientific method. Being a scientist requires a high level of correctness, with the clarity and humility to recognize biases and errors, and promptly address them.

And, most importantly, science is a social endeavor where the review and acceptance of research by colleagues and peers is of the essence. Scientists form an honor society where it is presupposed that it's members adhere to the scientific ethos of transparency, reproducibility, and respect of intellectual provenance, with the aim of building a cumulative, incremental body of knowledge.

As a scientist, I believe it is my responsibility to be consistently rational, credible, honorable, ethical, and keenly aware of my debt to society. And I expect my colleagues to hold me accountable to these standards as the price of admission to our community. This is not survival, quite the opposite: this is aspiring to an ideal, reaching out to a high standard of cognitive integrity for the betterment of humanity.

Lofty, yes. Easy, no. Worth it? Totally.

Monday, November 19, 2012

Infrasound Around the World, Hawaii to Palau

On a B777-200 aircraft to Guam, while sitting in the air strip I am measuring around 68 db per 1/3 octave band across the 6.3-20 Hz infrasound range, with all plane doors closed and air conditioning on. There was 6 db drop when AC went off temporarily. Sound levels were fairly steady, and I will concentrate on 10 Hz band levels for simplicity. Motoring on the ground and liftoff had highest levels, 90-100 db, ramping down to 72-77 db range on ascent and holding steady when we reached cruising altitude. The highest level reached during flight was 92 dB in the 40 Hz band, where there was a discrete peak. It's a bit of a challenge to make the measurements without freaking my fellow passers out or alarming the flight attendants. I walked around with the meter to see how homogeneous the levels were during the flight, and even in the bathroom the pressure levels were fairly even. The pressure levels during descent felt about the same. A dash through Guam to the next flight to Koror, which had a smaller plane and palpably lower ambient noise levels. There was no way to make these measurements discretely in this small plan, but it was my impression that that flight's sound had lower levels and a higher pitch.

Late arrival on Palau, and back up at 4 AM after a few good hours of sleep. I am at peace with sleeping only ~4 hours when on work travel, this is how my body responds to stress. That, and a ravenous appetite - eagerly waiting for breakfast.

Picture caption: Waiting for breakfast at that dawn. And there's coffee ...

Light breeze, maybe 1 m/s, ambient noise levels at 10 Hz fluctuating around 50-60 dB levels, depending on the wind gusts. Food is out, time to grind!


Sunday, November 18, 2012

Infrasound Around the World, HNL 121118

I'm about to board the first leg of a circumglobal trip, starting in Honolulu, Hawaii. I am bringing my trusty B&K Type 2250 hand-held sound analyzer, with the 4189 microphone, which will let me go down to 6.3 Hz and provide accurate sound pressure level (spl) estimates in 1/3 octave bands. So on 18 November at 2PM local time I can state with reasonable confidence that the spl at 6.3 Hz at the United Miles club is 55db, peaking around 65db at 10 Hz. I'll keep making routine logs as time and opportunity permits over the next 2.5 weeks.


Thursday, August 2, 2012

In the thick of sound

Wow, have not posted a thing since May? Yet words and symbols are at their most abundant: cranking out code, final proofs on a volcano acoustics book chapter, final edits on an ocean infrasound paper, two papers in progress on metrics and sources, and a fresh assignments with new customers.

Also since the last post, in order of appearance: field work in Diego Garcia, surfed Indo, nursed my brother through a jungle disease, nursed my wife through a popped knee, hosted an international experiment using the Mother of Infrasound Subwoofers (MOIS), remodeled the kitchen, picked up archery, quit drinking forever, and started consulting.

Papers, books, and instrumentation clutter every surface of my desk, stacks upon stacks of words, equations, and spectra, the imposed order of writing breeding chaos, inexorably increasing entropy. I no longer question why I persevere with this frantic pace, I understand it defines who I am.

Hope to come up for air sometime in late September, somewhere between London and Seoul.


Monday, May 14, 2012

The Mile High Ad Hoc Science Club

En route from Hawaii to the Indian Ocean I chanced into a crew of astronomers heading to an asteroid conference in Niigata, Japan. Dr. B. Werner, seatmate and seasoned astronomer from Colorado, enthusiastically and lucidly explained how the differential heating of irregular asteroids can lead to an increase in spin rate as they reradiate. And, most intriguingly, he suggested that there is a statistically significant level of repeatability in the alignment of their polar (rotation) axes perpendicular to the ecliptic, the plane where our planet revolves around the sun. Our conversation was prematurely interrupted by Dr. Werner's fortuitous seat upgrade, whereafter he was substituted by UH Astronomy Ph.D. student H. Kaluna, originally hailing from Pahoa, Big Island. From her I learned of the quest for water in asteroids, the techniques for inferring its possible presence, the potential relationship of such bodies to comets, and how they may hold insight on the origin of our Oceans. Her UH colleague and mentor, Dr. H. Hsieh smuggled beverages from first class, but he was chased away by ever-vigilant attendants and their armored carts before we could plumb the depths of his research.

Methinks engaging folks on their way to a meeting is a great way to learn new science: their results are freshly consolidated and they are primed to share them. But what struck me most was the friendliness and kindness of this crew, which triggered fond memories of my days as a stargazer. And I was impressed with the clarity and focus of Ms. Kaluna's planned career trajectory. A new generation of scientists appears cognizant that there may be more new Ph.D.'s being produced that there is a market in academia for them, and that other viable alternatives should also be considered.

On the road again: IO, Indian Ocean

Expedition preparations to remote areas are reliably intense - we don't get a second chance to prep, and we can't go to a local Radio Shack to get a fiber optic modem or a low-loss coax. So there is a sense of finality in boarding the first plane out: ready or not, he we go. The Chagos archipelago fits the definition of remoteness. Before boarding the first plane at the crack of dawn, we did get the good news that our on-island support crew is back on contract, a good thing as othwise we'd be hacking through the jungle with machetes for most of the trip. So off we go into the blue, via Japan, Singapore, and other vessels yet unknown.